Interesting idea, I too have had many thoughts and discussions on this topic.
We have seen many UFO reports over the years.
I am convinced that the majority of these are hoaxes/man-made phenomena, common astronomical/satellite observations, natural weather like ball lightning,etc.
However, some of them are too bizarre and too well documented to be so easily explained away.
If aliens have ever come here to visit the Earth, I'm sure that they marveled at the environment, weather patterns, and life ecosystems and microbiologies. The most obvious reason for a visit to our humble planet would be to collect samples of Earth life (specifically the DNA structures of various animals, plants, fungi, bacteria, viruses, etc.).
They would be very interested in studying the DNA of our planet if it was different than their home. They would also be fascinated to find if there were any similarities to their biology.
If they ever did make trips to Earth the treasures were probably not water, gold, or platinum but probably samples of life. They could make and learn a myriad of facts and uses from our planet's DNA.
We think about aliens getting in their space ship and cruising over here to check out things but if they were driving at the speeds our civilization travels it would take a looong time.
They would have to have found a method of traveling very fast or they would need to have the ability to travel in time.
I suppose they might download their consciousness into a "computer" and then have the consciousness transferred into a automation or clone upon arriving at a destination but time travel or near lightspeed travel might be more likely.
There might be at least several intelligent races capable of reaching our TIME and space and I'm sure if they come they must have an important reason and likley do not want to stop to chit chat (example: Star Trek prime directive).
I wouldn't be too suprised if we learned someday that tourist-like cruise ships came through from time to time.
The universe is very old and there is/has been/will be a lot of stuff going on out there that we are not aware of.
I read somewhere that if the observable galaxies in our universe were the size of peas..... that a large sports stadium could be filled by the peas.
If they do come to Earth there are many important reasons. With Earth's DNA they can build trees/plants, doctor stem cells, potentially alter their DNA to cope with a myriad of conditions, create ecosystems, create beneficial chemicals, engineer micro-organisms, make food, and so much more.